(1) Field of the Invention
This invention relates to a method of producing a hybrid multi-layered circuit of the type having rigid circuit substrates, each of which is subjected to a through-hole conductor formation treatment, affixed at a preselected location(s) and in registration with one another on both surfaces of a flexible circuit substrate. More particularly, the present invention is directed to a method of producing a hybrid multi-layered circuit wherein the opposing exposed surfaces of a flexible substrate sheet or sheets, having electrical circuitry previously provided thereon, are bonded to substantially inflexible copper-lined sheets in regions which are to be hybridized, the copper-lined sheets being subsequently transformed into rigid circuits, the method making it possible to carry out continuously through hole conductor formation and circuit pattern formation treatments at the hybridized portion of the flexible substrate without exerting any adverse influences on the flexible circuit substrate.
(2) Description of the Prior Art
Hybrid multi-layered circuit substrates are known in the art. Such hybrid circuit substrates offer enhanced component packaging density when compared to conventional flexible printed circuits. In a hybrid circuit, rigid circuit substrates are bonded to a flexible circuit substrate at a preselected location or locations. Hybrid multi-layered circuit substrates have generally been produced by the steps of separately preparing a flexible circuit substrate and rigid circuit substrate(s), bonding the rigid circuit substrate(s) to that portion of the flexible circuit substrate which is to be hybridized and multi-layered, and subsequently forming a pre-determined mutual conduction connection between the flexible circuit substrate and the rigid circuit substrate(s). The prior art technique, wherein the flexible and rigid circuit substrates were preformed, imposed serious limitations on the ability to interconnect the circuits, i.e., the conductive patterns, on the flexible and rigid substrates.
The difficulty in producing the desired interconnection pattern, in turn, seriously limited the ability to produce a hybrid multi-layered circuit substrate having high reliability and a high component packaging density. While it would be desirable to employ highly reliable, miniaturizable through-hole conductor means at the hybridized portion, i.e., where the flexible and rigid circuit substrates are bonded together, in actual fact complicated plating and masking techniques have had to be employed. Furthermore, in accordance with the prior art, considerable care must be taken to ensure against damage of the flexible circuit, both in the hybridized and unhybridized regions, during the process of producing the hybridized circuit portions and, particularly, during formation of the through-hole connections between the flexible and rigid substrates.
Japanese Patent Publication No. 12400/1986 discloses a method of producing a hybrid multi-layered circuit substrate wherein a rigid circuit substrate is laminated to a portion of a flexible circuit substrate while accomplishing sequentially the necessary electrical interconnection between the circuit patterns. The technique of the referenced Japanese patent publication includes the steps of forming through-holes in a base film which is employed for forming the flexible printed circuit substrate, through-hole plating of these holes, forming a pre-determined circuit pattern on one of the surfaces of the base film in association with the plated through-holes, disposing an adhesive layer in the region where the flexible circuit is to be hybridized and laminating a rigid substrate, the rigid substrate having separation perforations along a boundary line(s), to the base film. The boundary line(s) defined the region of the multi-layered circuit which were to be hybridized by permanent affixation of a rigid circuit substrate to the flexible circuit substrate comprising the base film. The technique being described also required that a resin be packed into the separation perforations in the rigid substrate. Thereafter, the electrical interconnection through-holes were drilled in the rigid substrate in the region which was to form part of the hybrid multi-layered circuit and these through-holes were subsequently plated. Next, the requisite circuit patterns were formed on the exposed surface of the rigid substrate in the multi-layered region and on the exposed surface of the base film in association with the through-holes. Finally, employing the separation perforations, the laminate member consisting of the flexible circuit substrate and bonded rigid circuit substrate is punched from the remaining laminate thus resulting in the removal of the rigid substrate in the non-multi-layered region. In accordance with the above-described method, it is possible to produce, through continuous steps, a hybrid multi-layered circuit substrate having a multi-layered region or regions consisting of flexible and rigid substrates and having though-hole connections between the circuit patterns on the flexible and rigid substrates, the hybridized circuit having non-multi-layered regions consisting of only the flexible circuit substrate.
In accordance with the hybrid multi-layered circuit production technique described above, however, the labor intensive step of packing the resin into the separation perforations that define the boundaries between the multi-layered region and non-multi-layer regions is required. Further, after the resin has been packed into these separation perforations, a further step of cleaning the rigid circuit substrate in the region of the separation perforations was required in order to permit the subsequent formation of a circuit pattern on the rigid substrate. The steps of packing the through-holes and cleaning the substrate complicate the method of manufacture. Also, and perhaps more importantly, the through-hole plating layer adheres to the circuit pattern on the flexible circuit substrate and this plating layer limits both mechanical flexibility and the flexibility in circuit design. A particularly significant limitation on the above-described method is that it precludes the use of a prefabricated flexible circuit substrate and thus, in many applications, increases the cost of production of a hybrid circuit.